Optical devices composed of planar optical waveguides in various configurations have much promise for optical communications systems. Silica waveguides, for example, have been configured into a variety of passive integrated optical circuits, and additional components have been added to make hybrid integrated circuits. An advantage of planar waveguides over fiber devices is that photolithography can be used to make a number of identical devices and circuits in a single processing sequence, much as electronic integrated circuits are made.
Bragg reflector filters are useful in a variety of integrated optical circuits, including drop filters and wavelength division multiplexers and demultiplexers. A typical Bragg filter comprises a length of optical waveguide having periodic perturbations in its index of refraction along its length to reflect light having a wavelength of twice the perturbation spacing. The perturbations can take the form of physical notches in the waveguide, its cladding, or both or can be photoinduced in the guiding material.
A difficulty in the fabrication of Bragg reflector filters arises because of the fine feature size required for wavelengths of interest for optical communication. Desired feature size of perturbations (.about.0.25 micrometer) requires either a state-of-the-art deep-UV stepper for photolithography or advanced non-lithographic processes such as holography or e-beam writing. The former approach uses exceedingly expensive equipment and the latter approaches forfeit the advantage of parallel processing inherent in photolithography. Accordingly, there is a need for an improved process for fabricating Bragg reflectors.